A little more than two decades ago, no planets had ever been detected outside the solar system. Now, more than 1,000 extrasolar planets have been confirmed, and on Monday the team behind the Kepler Space Telescope announced a haul of 833 more candidate planets to consider adding to the tally.

This embarrassment of riches is far beyond what scientists dared to hope for before NASA launched the Kepler mission in 2009. The telescope, in permanent orbit around the sun, identifies planets by watching them “transit,” or pass in front of, their stars, briefly dimming the stars’ light. “When I first started working with Kepler right before launch, I thought there would be maybe a thousand planets that Kepler would find,” Jason Rowe, an astronomer at the Search for Extraterrestrial Intelligence Institute in Mountain View, Calif., said during a press conference Monday at the Kepler Science Conference in Moffett Field, Calif.

In actuality, Kepler has uncovered more than 3,500 candidate exoplanets in its first three years, including large and small planets, rocky and gaseous worlds, and a total of 647 possible planets that appear to be Earth-sized. “We’re finding that there’s a wide variety of systems out there. If you can imagine it, the universe probably makes it,” Rowe said.

Scientists now know the universe does not lack for planets. Based on Kepler’s findings, researchers estimate our Milky Way galaxy alone could harbor around 140 billion planets. The burning question is, does life live on any of them? The best bet for finding life as we know it is to look for habitable environments that share the cushy offerings on Earth: plenty of liquid water, protective atmospheres, and stable, rocky construction with the right amount of sunshine. Astronomers have defined “the habitable zone” as a range of distances around stars, based on the stars’ temperatures, that are most likely to offer these amenities. So far Kepler has found 104 candidate planets that appear to reside in their stars’ habitable zones—10 of which are less than twice the radius of Earth. To know if any of these worlds really do have the right stuff for life, follow-up observations from next-generation telescopes will be needed.

Meanwhile, scientists are using Kepler’s statistics to get a better handle on general trends across the zoo of planets in the galaxy. Among stars similar to the sun, about 22 percent should host an Earth-sized planet in the habitable zone, according to study results announced Monday by Erik Petigura of the University of California, Berkeley. “The main question I think we’ve all asked is, how common is this ball of rock that we’re sitting on right now?” Petigura said at the press conference. “How rare or frequent is life in the universe? The results that are coming out of Kepler are an important milestone in answering that question.”

The Kepler telescope has ceased collecting data after a pair of failed reaction wheels hindered the spacecraft’s precision pointing ability in May. Sometime next year, NASA is expected to decide whether the telescope should take on a modified mission (there is no way to repair the broken components). In the meantime, astronomers still have a trove of data from Kepler’s fourth year of observation to parse for more planet signals. The most Earth-like planets—smaller worlds circling their stars in year-long orbits—are likely to be uncovered in this last batch of data because their slow orbits mean Kepler needed more time to observe their transits.

Ultimately, Kepler has just scratched the surface—it has observed only one-four hundredth of the sky. Yet within that small window, the observatory has found that about 70 percent of stars have planets, meaning the Milky Way is a crowded place. “If we ever get star travel, probably you’ll see a lot of traffic jams,” joked Kepler science principal investigator William Borucki of NASA’s Ames Research Center.

ABOUT THE AUTHOR(S)

Clara Moskowitz

Clara Moskowitz is Scientific American's senior editor covering space and physics. She has a bachelor's degree in astronomy and physics from Wesleyan University and a graduate degree in science journalism from the University of California, Santa Cruz.

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